1. Field of the Invention
The present invention relates to a color signal selection apparatus, color signal interpolation apparatus, color signal generation apparatus, surrounding point selection method, and a color conversion table creation method, which are particularly suitable for application to the selection of points surrounding an inputted point from points irregularly distributed on a space.
2. Description of the Related Art
Recently, the full-color image generation technology is rapidly developing in printers, facsimiles, etc., and there is a growing demand for sharing color images among different devices. However, an attempt to share color images among different devices presents the problem that, since the different devices provide different color reproducibilities, the colors of images sent from a transmission side are different from those of images obtained on a reception side.
In the case of printers and scanners, etc., for example, colors are respectively treated in the formats of color signals such as CMY signals and RGB signals, etc. which are sent to devices. In other words, in the case of printers having the values from 0 (minimum value) to 255 (maximum value) as the values of output color signals, all output colors are represented by combinations of CMY input signal values from 0 to 255. In this case, since even CMY signals with identical values provide different print colors, depending on printer models, it can be the that the representation of colors by CMY signals is device-dependent.
That is why color signals represented in a device-dependent representation format are transferred into an intermediate representation format which is independent of devices, thereby achieving the same color reproduction among different devices.
For example, the representations such as L*a*b signals and XYZ signals are used as methods for representing absolute colors which are independent of devices. These representations can be used to represent intermediate colors in the color conversion among the color spaces of different devices because colors represented by the representations are uniquely defined.
Specifically, images represented by RGB signals inputted from a scanner are converted into images represented by L*a*b* signals, and the L*a*b* signals are further converted into CMY signals before the images are outputted to a printer. Therefore, if the representation formats of these color signals are correctly converted, the same colors can be obtained among devices.
To convert the representation formats of color signals, the values of color signals represented in each format must be converted into the values of color signals of another format. One method for converting the values of color signals is to use a color conversion table describing the correspondence of colors in each color space. In the conversion of color signals by use of the color conversion table, when color signals registered in the color conversion table are converted into the color signals of another format, the correspondence registered in the color conversion table is used for the conversion. When a color signal not registered in the color conversion table is converted into the color signal of another format, color signals near the color signal to be converted are selected from the color signals registered in the color conversion table. These selected color signals are used for interpolation to convert the color signal to be converted.
Here, the color conversion table falls into two categories: a regularity table and an irregularity table. In the regularity table, the color signals to be converted are regularly distributed on a color space, whereas, in the irregularity table, the color signals to be converted are irregularly distributed on a color space.
In case of color conversion using the regularity table, even when a color signal not registered in the color conversion table has been inputted as one to-be-converted, color signals surrounding the color signal to-be-converted can be readily selected from among color signals registered in the color conversion table.
Moreover, since the selected color signals are distributed at grid points on a color space, the conversion of the color signal to-be-converted can be effected comparatively easily by a tetrahedron interpolation, triangle pole interpolation, cube interpolation, or the like.
On the other hand, in case of color conversion using the irregularity table, when a color signal not registered in the color conversion table has been inputted as one to-be-converted, this color signal to-be-converted is not always surrounded with selected color signals even by selecting the color signals near the color signal to-be-converted from among color signals registered in the color conversion table. In some cases, therefore, the precision of an interpolation is unsatisfactory in spite of the interpolation using the color signals near the color signal to-be-converted.
A method for selecting colors which are used for interpolative calculations in color conversion, and a method for transforming the layout of a color conversion table into a table which is easy of utilization in the execution of color conversion, are mentioned in the Official Gazette of Japanese Patent Application Laid-open No. 95431/1995 by way of example.
This example discloses a method wherein an interpolation is performed using data which are not distributed at grid points in a color space. That is, it states a color selection method in the case of color conversion using an irregularity table.
FIG. 1 is a flowchart showing the color selection method in the prior art.
Referring to the figure, k points x1 to xk nearest to a point x to-be-interpolated are selected (step ST1).
Subsequently, it is checked if the point x to-be-interpolated is surrounded with the selected k points x1 to xk (step ST2). On condition that the point x to-be-interpolated is not surrounded with the selected k points x1 to xk (step ST3), the next nearest points are selected (step ST4). The processing is iterated until points surrounding the point x to-be-interpolated is found. In due course, when the points surrounding the point x to-be-interpolated have been selected, the interpolation is performed using the selected points (step ST5).
Further, the afore cited Official Gazette of Japanese Patent Application Laid-open No. 95431/1995 refers to a method wherein, by employing a convex interpolation, a color conversion table (irregularity table) in which color signals are not distributed at grid points is transformed into a color conversion table (regularity table) in which color signals are distributed at grid points. With this method, the color signals of regular distribution are created by employing the convex interpolation, and the created color signals are registered in the regularity table.
With the color signal selection method, however, in the case where the point x to-be-interpolated is not surrounded with the selected k points x1 to xk, the selection of data is redone, and hence, the data selection needs to be iterated for the purpose of finding the points surrounding the point x to-be-interpolated. This leads to the problem of a long processing time period for selecting the points which surround the point x to-be-interpolated.
Another problem is that a time period is expended on the selection of surrounding color signals to lower the processing speed of color conversion, so a processing speed similarly lowers even in case of applying the surrounding color signals to the transformation of the irregularity table into the regularity table.
A further problem is that, in a case where each color signal to be transferred from the irregularity table to the regularity table is generated by interpolative calculations, it cannot be generated outside any range in which it is surrounded with the color signals registered in the irregularity table.